Conventional high frequency microwave ovens impart surface temperatures to foods of approximately 200.degree. F. which are insufficient to brown and crisp food products. Cooking temperatures in conventional convection and radiant ovens of 250.degree.-500.degree. F. are required for effective browning and crisping of foods. To simulate convection and radiant heat sources in microwave cooking, the food packaging industry has employed microwave susceptor materials which impart high temperature levels to food surfaces in the presence of microwave energy. Such susceptor materials have found wide application in providing disposable ovenable food containers made of paper and paperboard.
Conventional susceptor materials are fabricated by depositing a film of conductive elemental metal on a non-interactive or microwave transparent supporting substrate. Microwave energy interacts with the conductive metal coating to generate heat and provide a susceptor feature. However, conventional metalized films are not entirely satisfactory in that they require use of adhesives in the fabrication of the packaging. Such adhesives emit volatile chemicals in microwave applications and present health hazards. Thus adhesives require additional processing and safety specifications to meet FDA safety requirements.
As an alternative to conventional metalized films it has been proposed that microwave susceptor characteristics may be provided through use of chemical receptors. U.S. Pat. No. 4,283,427 to Winters et al. discloses microwave packaging materials which have a chemical susceptor layer comprising aqueous polar solvents, solutes including inorganic salts and heating profile monitor substances such as clay or silica.
Winters discloses microwave packaging materials in which a chemical susceptor layer is inserted within an enclosed plastic pouch. Upon heating in a microwave oven the aqueous polar solvent is vaporized leaving the solute material to heat to its maximum temperature to cook the food item. Heat profile monitors, are employed to control the rate at which the solvent vaporizes and the resultant temperature of the susceptor material. However, this approach is not entirely satisfactory in microwave applications in that the holder for the chemical susceptor must provide a means for escape of the solvent upon heating. If the solvent is not vented from the enclosed system, continuous heating of the food item occurs.
Further, attempts in the art to utilize chemical susceptors in microwave applications have been limited to coatings prepared and placed directly upon the food items to be cooked. See U.S. Pat. No. 4,518,618 to Hsia et al. and U.S. Pat. No. 4,252,832 to Moody.
Chemical susceptors comprising polar organic compounds have not, heretofore, been utilized in microwave food packaging materials. This invention is directed to the preparation of packaging materials which incorporate such polar organic compounds to provide desired thermal heating effects for food during microwave cooking. It will be appreciated that advantage would be obtained by providing such an alternative to metallic conductor or semi-conductor films as microwave susceptor materials.
Accordingly, it is a broad object of the invention to provide microwave susceptor materials which utilize polar organic compounds, having microwave interactive characteristics, coated or saturated on a substrate.
A more specific object of the invention is to provide a method which enhances the microwave interactivity of polar organic compounds through the addition of salts.
Another object of the invention is to provide low cost, flexible food packaging which can be used in microwave cooking that incorporates polar organic compounds imbided in various substrates.
A specific object of the invention is to provide microwave paper-like food packaging in which the type and relative amounts of polar organic compounds and salts may be varied to accommodate specific heat profile requirements of food products.